The invention relates generally to drilling of wells in subsurface formations. More specifically, the invention relates to monitoring and detecting kicks in a well.
During drilling of a well, drilling fluid (or “drilling mud”) is pumped from a mud pit into a drill string that is suspended in the well. The drilling mud flows down the drill string, exiting through a bit at the end of the drill string into the bottom of the well. The drilling mud then returns to the surface, carrying with it formation cuttings made by the bit. At the surface, the drilling mud flows through a mud return line into a mud treatment system, which cleans the drilling mud. The clean drilling mud is returned to the mud pit, from where the drilling mud is again pumped into the drill string. This circulation of the drilling mud continues while the bit is cutting the formation. The drilling mud performs a variety of functions, including carrying formation cuttings to the surface, cooling the bit, and controlling the hydrostatic pressure in the well such that the well does not take a kick. A well is said to take a kick whenever there is unwanted influx of formation fluids into the well.
The hydrostatic pressure in the well is controlled through the weight of the drilling mud. In spite of careful control of drilling mud weight, a well may take a kick unexpectedly. Thus the normal practice is to monitor the well for kicks so that as soon as a kick is detected measures can be put into place to circulate the kick out of the well and stabilize the well. If a kick is not detected early enough and controlled, it may result in blowout of the well. Strategies for detecting kicks generally include (i) monitoring increases in the difference between the volume of fluid pumped into the well and the volume of fluid returning from the well, (ii) monitoring increases in the difference between the rate at which fluid is pumped out of the well and the rate at which fluid is pumped into the well, (iii) monitoring fluctuations in drill pipe pressure, and (iv) monitoring increases in gas content of fluid returns from the well. It is common to use a combination of these strategies to effectively detect kicks during drilling operations.
The flow rate monitoring strategy is often used when drilling mud is not being pumped into the drill string, such as when making connections between drill pipes. The principle here is that if the well inflow is zero and the well is stable, the well outflow should also be zero. The current practice when using this strategy is to physically inspect the mud return line to confirm that flow stopped when the mud pump(s) stopped pumping drilling mud into the drill string. Rig personnel can look inside the bell nipple, which is a large diameter pipe at the top of the well to which the mud return line is attached, or further down the mud return line, such as at the shale shakers, to visually observe any signs of flow. However, when the rig crew first shuts the mud pump down, it generally takes some period of time for well outflow to drop to zero. To detect a kick early, the rig personnel inspecting the mud return line with the naked eye would need to be able to quickly distinguish between residual flow and anomalous flow that may be indicative of a kick.